Method for measuring channel state information in a wireless access system and apparatus for same

ABSTRACT

Disclosed are a method for measuring channel state information in a wireless access system that supports an environment in which an amount of uplink resource and an amount of downlink resource dynamically change, and an apparatus for the method. In detail, the method comprises: a step of receiving interference measurement resource information including information on the location of an interference measurement resource set in an uplink resource for interference measurement; a step of measuring interference being received from an adjacent cell at the location of the interference measurement resource a step of calculating channel state information using the measured interference value; and a step of transmitting the calculated channel state information to a base station.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/361,252, filed on May 28, 2014, now U.S. Pat. No. 9,467,881, which isthe National Stage filing under 35 U.S.C. 371 of InternationalApplication No. PCT/KR2012/011001, filed on Dec. 17, 2012, which claimsthe benefit of U.S. Provisional Application No. 61/576,356, filed onDec. 16, 2011, the contents of which are hereby incorporated byreference herein in their entirety.

TECHNICAL FIELD

The present invention relates to a wireless access system, and moreparticularly, a method for measuring channel state information in awireless access system for supporting an environment in which amounts ofuplink (UL) and downlink (DL) resources are dynamically changed, and anapparatus for supporting the method.

BACKGROUND ART

Mobile communication systems have been developed in order to providevoice services while ensuring the activity of users. However, a mobilecommunication system have gradually extended its field to data servicesas well as voice services and have been currently developed so as toprovide high speed data services. However, in a mobile communicationsystem that currently provides services, resources are insufficient andusers require higher speed services, and thus, there has been a need fora more developed mobile communication system.

One of most important factors among requirements of a next-generationwireless access system is to support high data transfer raterequirement. To this end, researches have been conducted into varioustechnologies such as multiple input multiple output (MIMO), cooperativemultiple point transmission (CoMP), relay, etc.

A conventional wireless access system, because uplink (UL) resources anddownlink (DL) resources are fixedly configured, even if UL and DLtraffic are changed, traffic is processed within limited resources.However, in consideration of an environment in which an eNB dynamicallychanges the amounts of UL and DL resources according to the amount of ULand DL traffic, even UL resource can be used as DL resource, and even DLresource can be used as UL resource. In this situation, even if resourceis configured for UL or DL, a UE needs to perform an appropriateoperation according to use of the corresponding resource.

DISCLOSURE Technical Problem

An object of the present invention devised to solve the problem lies ina method and apparatus for smoothly measuring a channel state between auser equipment (UE) and an eNB in a wireless access system, preferably,in a wireless access system for supporting an environment in whichamounts of uplink (UL) and downlink (DL) resources are dynamicallychanged, and an apparatus for the method.

Another object of the present invention devised to solve the problemlies in a method and apparatus for accurately measure a channel state ofresource that can be used for DL among resources configured as ULresources or a channel state of resource that can be used for UL amongresources configured as DL resources, and an apparatus for the method.

It is to be understood that both the foregoing general description andthe following detailed description of the present invention areexemplary and explanatory and are intended to provide furtherexplanation of the invention as claimed.

Technical Solution

The object of the present invention can be achieved by providing amethod for measuring channel state information by a user equipment (UE)in a wireless access system for supporting an environment in whichamounts of uplink (UL) and downlink (DL) resources are dynamicallychanged, the method including receiving interference measurementresource information including location information of interferencemeasurement resource configured as UL resource for interferencemeasurement, measuring interference exerted by an adjacent cell at alocation of the interference measurement resource, calculating thechannel state information using the measured interference, andtransmitting the calculated channel state information to an eNB.

In another aspect of the present invention, provided herein is a userequipment (UE) for measuring channel state information by a userequipment (UE) in a wireless access system for supporting an environmentin which amounts of uplink (UL) and downlink (DL) resources aredynamically changed, the UE including a radio frequency (RF) unit fortransmitting the channel state information and for transmitting andreceiving a radio signal, and a processor for receiving interferencemeasurement resource information including location information ofinterference measurement resource configured as UL resource forinterference measurement, for measuring interference exerted by anadjacent cell at a location of the interference measurement resource,calculating the channel state information using the measuredinterference, and transmitting the calculated channel state informationto an eNB.

The location information of the interference measurement resource mayinclude at least one of subframe offset, a subframe period, a subframeindex, and a symbol index.

The interference measurement resource may have any one form of ademodulation reference signal (DMRS), a sounding reference signal (SRS),a common reference signal (CRS), and a channel state informationreference signal (CSI).

The interference measurement resource may be semi-statically configuredvia a radio resource control (RRC) signal.

The method may further include receiving indication informationindicating that the interference measurement resource is valid.

The indication information may be an indicator for triggering aperiodicCSI reporting.

When a subframe included in the interference measurement resource isused for UL, nulling may be performed on a location of the interferencemeasurement resource in a UL channel transmission region of other UEsexcept for the UE and rate matching may be applied to data to betransmitted through the UL channel.

The interference measurement resource may be configured usinginformation about change in use of UL and DL of a subframe of an eNB ofan adjacent cell, received from the eNB of the adjacent cell.

Advantageous Effects

According to embodiments of the present invention, a channel statebetween a user equipment (UE) and an eNB can be smoothly measured in awireless access system, preferably, in a wireless access system forsupporting an environment in which amounts of uplink (UL) and downlink(DL) resources are dynamically changed.

According to the embodiments of the present invention, a channel stateof resource that can be used for DL among resources configured as ULresources or a channel state of resource that can be used for UL amongresources configured as DL resources can be accurately measured.

It will be appreciated by persons skilled in the art that that theeffects that could be achieved with the present invention are notlimited to what has been particularly described hereinabove and otheradvantages of the present invention will be more clearly understood fromthe following detailed description taken in conjunction with theaccompanying drawings.

DESCRIPTION OF DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention, illustrate embodiments of the inventionand together with the description serve to explain the principle of theinvention.

In the drawings:

FIG. 1 illustrates physical channels and a general method fortransmitting signals on physical channels in a 3^(rd) generationpartnership project long term evolution (3GPP LTE) system;

FIG. 2 illustrates a structure of a radio frame used in a 3GPP LTE;

FIG. 3 illustrates the structure of a downlink (DL) resource grid forthe duration of one DL slot;

FIG. 4 illustrates a structure of a DL subframe;

FIG. 5 illustrates a structure of an uplink (UL) subframe;

FIG. 6 is a diagram illustrating an example of a channel stateinformation measuring method according to an embodiment of the presentinvention;

FIG. 7 is a schematic diagram illustrating a case in which a subframeincluding interference measurement resource is used for UL according toan embodiment of the present invention;

FIG. 8 is a schematic diagram illustrating a case in which a subframeincluding interference measurement resource is used for DL according toan embodiment of the present invention; and

FIG. 9 is a block diagram of a wireless communication apparatusaccording to an embodiment of the present invention.

BEST MODE

Reference will now be made in detail to the exemplary embodiments of thepresent invention with reference to the accompanying drawings. Thedetailed description, which will be given below with reference to theaccompanying drawings, is intended to explain exemplary embodiments ofthe present invention, rather than to show the only embodiments that maybe implemented according to the invention. The following detaileddescription includes specific details in order to provide a thoroughunderstanding of the present invention. However, it will be apparent tothose skilled in the art that the present invention may be practicedwithout such specific details.

In some instances, well-known structures and devices are omitted inorder to avoid obscuring the concepts of the present invention andimportant functions of the structures and devices are shown in blockdiagram form.

The embodiments of the present invention are disclosed on the basis of adata communication relationship between a base station and a terminal.In this case, the base station is used as a terminal node of a networkvia which the base station can directly communicate with the terminal.Specific operations to be conducted by the base station in the presentinvention may also be conducted by an upper node of the base station asnecessary. In other words, it will be obvious to those skilled in theart that various operations for enabling the base station to communicatewith the terminal in a network composed of several network nodesincluding the base station will be conducted by the base station orother network nodes other than the base station. The term “base station(BS)” may be replaced with a fixed station, Node-B, eNode-B (eNB), or anaccess point (AP) as necessary. The term “relay” may be replaced withthe terms relay node (RN) or relay station (RS). The term “terminal” mayalso be replaced with a user equipment (UE), a mobile station (MS), amobile subscriber station (MSS), a subscriber station (SS), an advancedmobile station (AMS), a wireless terminal (WT), a machine-typecommunication (MTC) device, a machine-to-machine (M2M) device, or adevice-to-device (D2D) device as necessary.

It should be noted that specific terms disclosed in the presentinvention are proposed for convenience of description and betterunderstanding of the present invention, and the use of these specificterms may be changed to other formats within the technical scope orspirit of the present invention.

Exemplary embodiments of the present invention are supported by standarddocuments disclosed for at least one of wireless access systemsincluding an institute of electrical and electronics engineers (IEEE)802 system, a 3^(rd) generation partnership project (3GPP) system, a3GPP long term evolution (LTE) system, an LTE-advanced (LTE-A) system,and a 3GPP2 system. In particular, steps or parts, which are notdescribed to clearly reveal the technical idea of the present invention,in the embodiments of the present invention may be supported by theabove documents. All terminology used herein may be supported by atleast one of the above-mentioned documents.

The following embodiments of the present invention can be applied to avariety of wireless access technologies, for example, code divisionmultiple access (CDMA), frequency division multiple access (FDMA), timedivision multiple access (TDMA), orthogonal frequency division multipleaccess (OFDMA), single carrier frequency division multiple access(SC-FDMA), and the like. CDMA may be embodied through wireless (orradio) technology such as universal terrestrial radio access (utra) orCDMA2000. TDMA may be embodied through wireless (or radio) technologysuch as global system for mobile communication (GSM)/general packetradio service (GPRS)/enhanced data rates for GSM evolution (EDGE). OFDMAmay be embodied through wireless (or radio) technology such as instituteof electrical and electronics engineers (IEEE) 802.11 (Wi-Fi), IEEE802.16 (WiMAX), IEEE 802-20, and evolved UTRA (E-UTRA). UTRA is a partof universal mobile telecommunications system (UMTS). 3rd generationpartnership project (3GPP) long term evolution (LTE) is a part of E-UMTS(Evolved UMTS), which uses E-UTRA. 3GPP LTE employs OFDMA in downlinkand employs SC-FDMA in uplink. LTE-Advanced (LTE-A) is an evolvedversion of 3GPP LTE.

For clarity, the following description focuses on IEEE 802.11 systems.However, technical features of the present invention are not limitedthereto.

1. 3GPP LTE/LTE-A System to which the Present Invention is Applicable

1. 1. Overview of System

FIG. 1 illustrates physical channels and a general method fortransmitting signals on the physical channels in the 3GPP LTE system.

When a UE is powered on or enters a new cell, the UE performs initialcell search (S101). The initial cell search involves acquisition ofsynchronization to an eNB. Specifically, the UE synchronizes its timingto the eNB and acquires information such as a cell identifier (ID) byreceiving a primary synchronization channel (P-SCH) and a secondarysynchronization channel (S—SCH) from the eNB.

Then the UE may acquire information broadcast in the cell by receiving aphysical broadcast channel (PBCH) from the eNB. During the initial cellsearch, the UE may monitor a DL channel state by receiving a downlinkreference signal (DL RS).

After the initial cell search, the UE may acquire more detailed systeminformation by receiving a physical downlink control channel (PDCCH) andreceiving a physical downlink shared channel (PDSCH) based oninformation of the PDCCH (S102).

To complete access to the eNB, the UE may perform a random accessprocedure with the eNB (S103 to S106). In the random access procedure,the UE may transmit a preamble on a physical random access channel(PRACH) (S103) and may receive a response message to the preamble on aPDCCH and a PDSCH associated with the PDCCH (S104). In the case of acontention-based random access, the UE may additionally perform acontention resolution procedure including transmission of an additionalPRACH (S105) and reception of a PDCCH signal and a PDSCH signalcorresponding to the PDCCH signal (S106).

After the above procedure, the UE may receive a PDCCH and/or a PDSCHfrom the eNB (S107) and transmit a Physical Uplink Shared Channel(PUSCH) and/or a Physical Uplink Control Channel (PUCCH) to the eNB(S108), in a general UL/DL signal transmission procedure.

Control information that the UE transmits to the eNB is called uplinkcontrol information (UCI). The UCI includes hybrid automatic repeat andrequest acknowledgement/negative acknowledgement (HARQ-ACK/NACK),scheduling request (SR), channel quality indication (CQI), precodingmatrix index (PMI), a rank indication (RI), etc.

In the LTE system, UCI is generally transmitted on a PUCCH periodically.However, if control information and traffic data should be transmittedsimultaneously, they may be transmitted on a PUSCH. In addition, UCI maybe transmitted periodically on the PUSCH, upon receipt of arequest/command from a network.

FIG. 2 illustrates a structure of a radio frame used in a 3GPP LTE.

In a cellular orthogonal frequency division multiplexing (OFDM) wirelesspacket communication system, UL/DL data packets are transmitted insubframes. One subframe is defined as a predetermined time intervalincluding a plurality of OFDM symbols. The 3GPP LTE standard supports atype 1 radio frame structure applicable to frequency division duplex(FDD) and a type 2 radio frame structure applicable to time divisionduplex (TDD).

FIG. 2(a) is a diagram illustrating the structure of the type 1 radioframe. A DL radio frame includes 10 subframes, each subframe includingtwo slots in the time domain. A time required for transmitting onesubframe is defined as a transmission time interval (TTI). For example,one subframe may be 1 ms long and one slot may be 0.5 ms long. One slotincludes a plurality of OFDM symbols in the time domain and a pluralityof resource blocks (RBs) in the frequency domain. Since the 3GPP LTEsystem uses OFDMA for DL, an OFDM symbol may be one symbol period. TheOFDM symbol may be called an SC-FDMA symbol or symbol period. An RB is aresource allocation unit including a plurality of contiguous subcarriersin one slot.

The number of OFDM symbols included in one slot may be changed accordingto the configuration of a cyclic prefix (CP). There are two types ofCPs, extended CP and normal CP. For example, if each OFDM symbol isconfigured to include a normal CP, one slot may include 7 OFDM symbols.If each OFDM symbol is configured to include an extended CP, the lengthof an OFDM symbol is increased and thus the number of OFDM symbolsincluded in one slot is less than that in the case of a normal CP. Inthe case of the extended CP, for example, one slot may include 6 OFDMsymbols. If a channel state is instable as is the case with a fast UE,the extended CP may be used in order to further reduce inter-symbolinterference.

In the case of the normal CP, since one slot includes 7 OFDM symbols,one subframe includes 14 OFDM symbols. Up to first three OFDM symbols ofeach subframe may be allocated to a PDCCH and the remaining OFDM symbolsmay be allocated to a PDSCH.

FIG. 2(b) illustrates the structure of the type 2 radio frame. The type2 radio frame includes two half frames, each half frame including 5subframes, a downlink pilot time slot (DwPTS), a guard period (GP), andan uplink pilot time slot (UpPTS). One subframe is divided into twoslots. The DwPTS is used for initial cell search, synchronization, orchannel estimation at a UE, and the UpPTS is used for channel estimationand UL transmission synchronization with a UE at an eNB. The GP is usedto cancel UL interference between UL and DL, caused by the multi-pathdelay of a DL signal.

UL-DL configuration of the type 2 radio frame structure of a TDD systemrefers to a rule indicating whether UL and DL are allocated (orreserved) to all subframes. Table 1 shows an exemplary uplink-downlinkconfiguration.

TABLE 1 Uplink- Downlink- downlink to-Uplink config- Switch-pointSubframe number uration periodicity 0 1 2 3 4 5 6 7 8 9 0 5 ms D S U U UD S U U U 1 5 ms D S U U D D S U U D 2 5 ms D S U D D D S U D D 3 10 ms D S U U U D D D D D 4 10 ms  D S U U D D D D D D 5 10 ms  D S U D D D DD D D 6 5 ms D S U U U D S U U D

In Table 3 above, for each respective subframe of a radio frame, “D”denotes a downlink subframe, “U” denotes an uplink subframe, and “S”denotes a special subframe including three fields of a DwPTS, a GP, anda an UpPTS. The UL-DL configuration may be classified into 7 types, andfor each respective configurations, the locations and numbers of DLsubframes, special subframes, and UL subframes are varied.

A point in time for converting DL into UL or a pint in time forconverting UL into DL is referred to as a switching point. Aswitch-point periodicity refers to a period with which an operation ofconversion between a UL subframe and a DL subframe is repeated andsupports both 5 ms and 10 ms. When the switch-point periodicity is 5 ms,a special subframe S is present every half-frame. When the switch-pointperiodicity is 10 ms, a special subframe S is present only in a firsthalf-frame.

In all configurations, subframes #0 and #5 and a DwPTS are period for DLtransmission only. The UpPTS, a subframe, and a subframe immediatelysubsequent thereto are always periods for UL transmission.

The UL-DL configuration may be system information and may be known toboth an eNB and a UE. Whenever UL-DL configuration information ischanged, the eNB may transmit only an index of configuration informationto notify the UE of information about change in UL-DL allocation stateof a radio frame. In addition, the configuration information may betransmitted as a type of DL control information through a physicaldownlink control channel (PDCCH) and may be commonly transmitted as atype of broadcast information to all UEs in a cell through a broadcastchannel like other scheduling information and

The aforementioned radio frame structure is purely exemplary. The numberof subframes included in a radio frame or the number of slots includedin each subframe, and the number of symbols of each slot can be changedin various ways.

FIG. 3 illustrates the structure of a DL resource grid for the durationof one DL slot.

Referring to FIG. 3, one DL slot includes a plurality of OFDM symbols inthe time domain Here, one DL slot includes 7 OFDM symbols and oneresource block includes 12 subcarriers in the frequency symbol, which ispurely exemplary, but embodiments of the present invention are notlimited thereto.

Each element of the resource grid is referred to as a resource element(RE). An RB includes 12×7 REs. The number of RBs in a DL slot, N^(DL)depends on a DL transmission bandwidth. A UL slot may have the samestructure as a DL slot.

FIG. 4 illustrates a structure of a DL subframe.

Referring to FIG. 4, up to three or four OFDM symbols at the start ofthe first slot of a DL subframe are used as a control region to whichcontrol channels are allocated and the other OFDM symbols of the DLsubframe are used as a data region to which a PDSCH is allocated. DLcontrol channels defined for the 3GPP LTE system include a physicalcontrol format indicator channel (PCFICH), a physical downlink controlchannel (PDCCH), and a physical hybrid-ARQ indicator channel (PHICH).

The PCFICH is located in the first OFDM symbol of a subframe, carryinginformation about the number of OFDM symbols used for transmission ofcontrol channels in the subframe. The PHICH delivers an HARQ ACK/NACKsignal as a response to a UL transmission. Control information carriedon the PDCCH is called downlink control information (DCI). The DCItransports resource allocation information and other control informationfor a UE or a UE group. For example, the DCI includes DL/UL schedulinginformation, UL transmission (Tx) power control commands, etc.

The PDCCH delivers information about resource allocation and a transportformat for a downlink shared channel (DL-SCH), information aboutresource allocation and a transport format for an uplink shared channel(UL-SCH), paging information of a paging channel (PCH), systeminformation on the DL-SCH, information about resource allocation for ahigher-layer control message such as a random access responsetransmitted on the PDSCH, a set of Tx power control commands forindividual UEs of a UE group, Tx power control commands, voice overInternet protocol (VoIP) activation indication information, etc. Aplurality of PDCCHs may be transmitted in the control region. A UE maymonitor a plurality of PDCCHs. A PDCCH is transmitted in an aggregate ofone or more consecutive Control Channel Elements (CCEs). A CCE is alogical allocation unit used to provide a PDCCH at a coding rate basedon the state of a radio channel. A CCE includes a plurality of RE groups(REGs). The format of a PDCCH and the number of available bits for thePDCCH are determined according to the number of CCEs and a coding rateprovided by the CCEs.

An eNB determines a PDCCH format according to DCI transmitted to a UEand adds a cyclic redundancy check (CRC) to control information. The CRCis masked by an identifier (ID) known as a radio network temporaryidentifier (RNTI) according to the owner or usage of the PDCCH. If thePDCCH is destined for a specific UE, the CRC may be masked by acell-RNTI (C-RNTI) of the UE. If the PDCCH carries a paging message, theCRC of the PDCCH may be masked by a paging indicator identifier(P-RNTI). If the PDCCH carries system information, particularly, asystem information block (SIB), its CRC may be masked by a systeminformation ID and a system information RNTI (SI-RNTI). To indicate thatthe PDCCH carries a random access response to a random access preambletransmitted by a UE, its CRC may be masked by a random access-RNTI(RA-RNTI).

FIG. 5 illustrates a structure of a UL subframe.

Referring to FIG. 5, a UL subframe may be divided into a control regionand a data region in the frequency domain. The control region includes aPUCCH that carriers UL control information. The data region includes aPUSCH that carrier user data. In order to maintain single carrier waveproperties, one UE may not simultaneously transmit a PUCCH and a PUSCH.An RB pair is allocated to a PUCCH of one UE in a subframe. RBs includedin an RB pair occupy different subcarriers in two respective slots. TheRB pair allocated to the PUCCH frequency-hops over a slot boundary.

1. 2. DL Measurement

In a mobile communication system, a packet (or signal) is transmitted ona radio channel from a transmitter to a receiver. In view of the natureof the radio channel, the packet may be distorted during thetransmission. To receive the signal successfully, the receiver shouldcompensate for the distortion in the received signal using channelinformation. Generally, to enable the receiver to acquire the channelinformation, the transmitter transmits a signal known to both thetransmitter and the receiver and the receiver acquires knowledge ofchannel information based on the distortion of the signal received onthe radio channel. The signal known to both the transmitter and receiveris referred to as a pilot signal or a reference signal (RS).

In transmission and reception of data using multiple antennas, thereceiver needs to know channel states between transmit antennas andreceive antennas to successfully receive a signal. Accordingly, aseparate reference signal is needed for each transmit antenna.

In a wireless communication system, an RS can be largely classified intotwo types according to its purpose. The RS includes an RS for channelinformation acquisition and an RS for data demodulation. The former isused for acquisition of channel information to DL by a UE. Thus theformer RS needs to be transmitted in a wideband, and even a UE that doesnot receive DL data in a specific subframe needs to receive and measurethe RS. In addition, the RS for channel measurement may also be used formeasurement of handover, etc. The latter is an RS that is transmittedtogether with corresponding resource when an eNB transmits a DL signal.In this regard, the UE can receive the corresponding RS to estimate achannel and accordingly demodulate data. The RS for data demodulationneeds to be transmitted in a region in which data is transmitted.

A 3GPP LTE system defines a common reference signal (CRS) shared by allUEs in a cell and a dedicated reference signal (DRS) for a specific UEonly as a DL RS. The CRS may be used for both channel informationacquisition and data demodulation and may also be referred to as acell-specific RS. An eNB transmits the CRS every subframe over awideband. On the other hand, the DRS may be used for data demodulationonly and may be transmitted through REs when data modulation on a PDSCHis required. The UE may receive whether the DRS is present through ahigher layer and determines that the DRS is valid only when thecorresponding PDSCH is mapped. The DRS may be referred to as aUE-specific RS or a demodulation RS (DMRS).

A receiver (UE) may estimate a channel state from the CRS and feedbackan indicator associated with channel quality, such as a channel qualityindicator (CQI), a precoding matrix index (PMI), and/or a rank indicator(RI) to a transmitter (eNB). In addition, the receiver may define an RSassociated with feedback of channel state information (CSI) such asCQI/PMI/RI as a separate CSI-RS. A CSI-RS for channel measurement isdesigned mainly for channel measurement unlike an existing CRS used fordata demodulation as well as channel measurement, etc. Since the CSI-RSis transmitted only for transmission of information about a channelstate, the eNB transmits CSI-RSs about all antenna ports. In addition,the CSI-RS is transmitted for knowledge of DL channel information andthus is transmitted over all bands unlike a DRS.

A current 3GPP LTE system defines two types of a closed-loop MIMOtransmission scheme and an open-loop MIMO scheme managed without channelinformation of the receiver. In the closed-loop MIMO, in order toachieve multiplexing gain of a MIMO antenna, each of the transmitter andthe receiver performs beamforming based on channel information, that is,channel state information (CSI). The eNB may command the UE to allocatea physical uplink control channel (PUCCH) or a physical uplink sharedchannel (PUSCH) and to feedback DL CSI in order to acquire CSI from theUE.

CSI is classified largely into three information types, a rank indicator(RI), a precoding matrix index (PMI), and a channel quality indication(CQI).

An RI is information about a channel rank that is the number of signalstreams (or layers) that a UE can receive in the same time-frequencyresources. Because the RI is determined dominantly according to thelong-term fading of a channel, the RI may be fed back to an eNB in alonger period than a PMI and a CQI.

A PMI is the index of a UE-preferred eNB precoding matrix determinedbased on a metric such as signal to interference and noise ratio (SINR),reflecting the spatial characteristics of channels. The PMI reflectschannel spatial characteristics and indicates a precoding index of aneNB preferred by a UE based on a metric such as a signal to interferenceplus noise ratio (SINR), etc. That is, the PMI is information about aprecoding matrix used for transmitted from a transmitter. The precodingmatrix fed back from a received is determined in consideration of thenumber of a layer indicted by an RI. The PMI may be fed back in case ofclosed-loop spatial multiplexing (SM) and large delay cyclic delaydiversity (CDD). In the case of open-loop transmission, the transmittermay select a precoding matrix according to predetermined rules. Aprocess for selecting a PMI for each rank is as follows. The receivermay calculate a post processing SINR in each PMI, convert the calculatedSINR into the sum capacity, and select the best PMI on the basis of thesum capacity. That is, PMI calculation of the receiver may be consideredto be a process for searching for an optimum PMI on the basis of the sumcapacity. The transmitter that has received PMI feedback from thereceiver may use a precoding matrix recommended by the receiver. Thisfact may be contained as a 1-bit indicator in scheduling allocationinformation for data transmission to the receiver. Alternatively, thetransmitter may not use the precoding matrix indicated by a PMI fed backfrom the transmitter. In this case, precoding matrix information usedfor data transmission from the transmitter to the receiver may beexplicitly contained in the scheduling allocation information.

A CQI represents a channel strength and in general reflects a receptionSINR that the eNB can achieve with a PMI. A UE reports CQI index to aneNB. The CQI index indicates a specific combination of a set includingcombination of a predetermined modulation scheme and code rate.

In an evolved communication system such as LTE-A, additional multi-userdiversity gain is obtained using multi-user MIMO (MU-MIMO). The MU-MIMOtechnology refers to a method of a scheme in which an eNB assignsantenna resources to different UEs and selects and schedules a UE thatcan have a high data transfer rate for each antenna. For the multi-userdiversity gain, higher accuracy is required from a viewpoint of achannel feedback. Since interference is present between UEs multiplexedin the antenna domain in MU-MIMO, accuracy of CSI may largely affect notonly a UE that reports the CSI but also interference of othermultiplexed UEs. Accordingly, in order to enhance the accuracy of afeedback channel in LTE-A system, a final PMI may be determined to bedivided into W1 corresponding to a long-term and/or wideband PMI and W2corresponding to a short-term and/or subband PMI and may be determinedas a combination of W1 and W2.

For example, the long-term covariance matrix of channels expressed as[Equation 1] may be used for hierarchical codebook transformation thatconfigures one final PMI with W1 and W2 from information of twochannels.W=norm(W1W2)  [Equation 1]

In [Equation 1], W2 is a short-term PMI, which is a codeword of acodebook reflecting short-term channel information, W1 is a long-termcovariance matrix, and norm(A) is a matrix obtained by normalizing thenorm of each column of matrix A to 1. W is a codeword of a finaltransformed codebook. Conventionally, W1 and W2 are given according to[Equation 2] below.

$\begin{matrix}{{{W\; 1(i)} = \begin{bmatrix}X_{i} & 0 \\0 & X_{i}\end{bmatrix}},{{{where}\mspace{14mu} X_{i}\mspace{14mu}{is}\mspace{14mu}{{Nt}/2}\mspace{14mu}{by}\mspace{14mu} M\mspace{14mu}{{matrix}.W}\; 2(j)} = {\overset{\overset{r\mspace{14mu}{columns}}{︷}}{\begin{bmatrix}e_{M}^{k} & e_{M}^{l} & \; & e_{M}^{m} \\{\alpha_{j}e_{M}^{k}} & {\beta_{j}e_{M}^{l}} & \ldots & {\gamma_{j}e_{M}^{m}}\end{bmatrix}\mspace{11mu}}\;\left( {{{if}\mspace{14mu}{rank}} = r} \right)}},{{{where}\mspace{14mu} 1} \leq k},l,{m \leq {M\mspace{14mu}{and}\mspace{14mu} k}},l,{m\mspace{14mu}{are}\mspace{14mu}{{integer}.}}} & \left\lbrack {{Equation}\mspace{14mu} 2} \right\rbrack\end{matrix}$

In [Equation 2] above, the codewords are designed so as to reflectcorrelation characteristics between established channels, if crosspolarized antennas are arranged densely (for example, the distancebetween adjacent antennas is equal to or less than a half of a signalwavelength. The cross polarized antennas may be divided into ahorizontal antenna group and a vertical antenna group and the twoantenna groups are co-located, each having the property of a uniformlinear array (ULA) antenna. Therefore, the correlations between antennasin each group have the same linear phase increment property and thecorrelation between the antenna groups is characterized by phaserotation. Since a codebook is eventually quantized values of channels,it is necessary to design a codebook, reflecting channelcharacteristics. For the convenience of description, a rank-1 codeworddesigned according to [Equation 2] may be given as [Equation 3] below.

$\begin{matrix}{{W\; 1(i)*W\; 2(j)} = \begin{bmatrix}{X_{i}(k)} \\{\alpha_{j}{X_{i}(k)}}\end{bmatrix}} & \left\lbrack {{Equation}\mspace{14mu} 3} \right\rbrack\end{matrix}$

In [Equation 3] above, a codeword is expressed as an N_(T)×1 vectorwhere NT is the number of Tx antennas and the codeword is composed of anupper vector X_(i)(k) and a lower vector α_(j)X_(i)(k), representing thecorrelation characteristics of the horizontal and vertical antennagroups, respectively. Preferably, X_(i)(k) is expressed as a vectorhaving the linear phase increment property, reflecting the correlationcharacteristics between antennas in each antenna group. For example, adiscrete Fourier transform (DFT) matrix may be used for X_(i)(k).

Higher accuracy is required for CoMP. In the case of CoMP JT, aplurality of eNBs collaboratively transmits the same data to a specificUE, and thus, a CoMP JT system may be academically considered as a MIMOsystem in which antennas are geographically distributed. That is, whenthe JT performs MU-MIMO, high level channel accuracy is also required inorder to prevent co-scheduled UEs like single cell MU-MIMO. In the caseof CoMP CB, accurate channel information is also required in order toprevent interference to a serving cell by an adjacent cell.

Recently, active research has been conducted into enhanced inter-cellinterference coordination (eICIC) as an interference coordination methodbetween UEs in a 3GPP LTE-A system. The eICIC is one of interferencecoordination methods. In this regard, according to the eICIC, a cellcausing interference is defined as an aggressor cell or a primary cell,an interfered cell is defined as a victim cell or a secondary cell, theaggressor cell stops data transmission in some specific resource regionssuch that a UE can maintain access to the victim cell or secondary cellin the corresponding resource region. That is, time domain inter-cellinterference coordination by which an aggressor cell uses a silentsubframe that reduces transmission power/activity of some physicalchannels (including operation of setting zero power) and a victim cellschedules UEs in consideration of the silent frame can be used. Thesilent subframe may also be called an almost blank subframe (ABS). Inthis case, from a viewpoint of a UE positioned in the victim cell, aninterference level largely changes according to whether the silentsubframe is present, and signals transmitted from the aggressor cell andthe victim cell may act as interference to a UE positioned at a boundarybetween the aggressor cell and the victim cell.

In this situation, to perform more accurate radio link monitoring (RLM)in each subframe or radio resource management (RRM) for measuringreference signal received power (RSRP)/reference signal received quality(RSRQ) or to measure channel state information (CSI) for linkadaptation, the aforementioned monitoring/measurement needs to belimited to subframe sets having uniform interference characteristics.

In 3GPP LTE system, the following restricted RLM and RRM/CSI measurementis defined.

1) RLM

The DL radio link quality may be monitored by a physical layer of a UEin order to indicate an ‘out-of-sync’ or ‘in-sync’ status to higherlayers.

In the case of a non-discontinuous reception (DRX) mode operation, thephysical layer in the UE compares a value measured over a previous timeperiod every radio frame with thresholds (Q_(out) and Q_(in)) to monitorradio link quality. On the other hand, in the case of a DRX modeoperation, the physical layer in the UE compares a value measured over aprevious time period every DRX period at least once to monitor radiolink quality. Here, if higher layer signaling indicates specificsubframes for restricted radio link monitoring, the radio link qualityis not monitored by other subframes other than the indicated subframes.

The physical layer in the UE indicates ‘out-of-sync’ to higher layerswhen the radio link quality is worse than the threshold Q_(out) in radioframes in which the radio link quality is assessed. That is, the‘out-of-sync’ indication is an event that occurs when a UE measures thechannel quality of a signal from a serving eNB and the channel qualityis degraded to a predetermined level or less. Here, the channel qualitymay be measured from a signal-to-noise ratio (SNR) measured using acell-specific reference signal (CRS) of a DL signal from the eNB. Inaddition, the ‘out-of-sync’ indication may be provided to higher layerswhen a PDCCH received from lower layers (physical layers) cannot bedemodulated or signal-to-interference plus noise ratio (SINR) is low.

On the other hand, when the physical layer in the UE is better than thethreshold Q_(in) in radio frames in which the radio link quality isassessed, ‘in-sync’ is indicated to higher layers. That is, the‘in-sync’ indication is an event that occurs when a UE measures thechannel quality of a signal from a serving eNB and the channel qualityis increased to a predetermined level or more.

2) Channel Quality Indicator (CQI)

CQI is information regarding channel quality. CQI may be represented bya predetermined MCS combination. CQI index may be given as shown inTable 2 below.

Table 2 shows CQI index.

TABLE 2 CQI index modulation code rate × 1024 efficiency 0 out of range1 QPSK 78 0.1523 2 QPSK 120 0.2344 3 QPSK 193 0.3770 4 QPSK 308 0.6016 5QPSK 449 0.8770 6 QPSK 602 1.1758 7 16QAM 378 1.4766 8 16QAM 490 1.91419 16QAM 616 2.4063 10 64QAM 466 2.7305 11 64QAM 567 3.3223 12 64QAM 6663.9023 13 64QAM 772 4.5234 14 64QAM 873 5.1152 15 64QAM 948 5.5547

Table 3 below shows a PDSCH transmission scheme for CSI referenceresource.

TABLE 3 Transmis- sion mode Transmission scheme of PDSCH 1Single-antenna port, port 0 2 Transmit diversity 3 Transmit diversity ifthe associated rank indicator is 1, otherwise large delay CDD 4Closed-loop spatial multiplexing 5 Multi-user MIMO 6 Closed-loop spatialmultiplexing with a single transmission layer 7 If the number of PBCHantenna ports is one, Single-antenna port, port 0; otherwise Transmitdiversity 8 If the UE is configured without PMI/RI reporting: if thenumber of PBCH antenna ports is one, single-antenna port, port 0;otherwise transmit diversity If the UE is configured with PMI/RIreporting: closed-loop spatial multiplexing 9 If the UE is configuredwithout PMI/RI reporting: if the number of PBCH antenna ports is one,single-antenna port, port 0; otherwise transmit diversity Closed-loopspatial multiplexing with up to 8 layer transmission, ports 7-14 (seesubclause 7.1.5B)

Referring to Table 2 above, CQI index may be represented by 4 bits(i.e., CQI indexes of 0-15). Each CQI index may indicate a modulationscheme and a code rate.

A 3GPP LTE/LTE-A system defines that the following assumptions areconsidered in order to calculate CQI index by a UE from CSI referenceresource.

(1) The first three OFDM symbols in one subframe are occupied by controlsignaling.

(2) Resource elements (REs) used by a primary synchronization signal, asecondary synchronization signal or a physical broadcast channel (PBCH)are not present.

(3) CP length of a non-MBSFN subframe is assumed.

(4) Redundancy version is set to zero (0).

(5) In the case of CSI reporting in transmission mode 9, when a UEconfigures PMI/RI reporting, DMRS overhead is the same as most recentlyreported rank.

(6) REs used for CSI-RS and zero-power CSI-RS are not present.

(7) REs used for Positioning Reference Signal (PRS) are not present.

(8) PDSCH transmission method may be dependent upon a currenttransmission mode (e.g., a default mode) configured in a UE and givenaccording to Table 3 above.

(9) The ratio of PDSCH EPRE (energy per resource element) to acell-specific reference signal EPRE may be given with the exception ofρ_(A). (A detailed description of ρ_(A) may follow the followingassumption. Provided that a UE for an arbitrary modulation scheme may beset to transmission mode 2 having four cell-specific antenna ports ormay be set to transmission mode 3 having an RI of 1 and fourcell-specific antenna ports, ρ_(A) may be denoted byρ_(A)=P_(A)+Δ_(offset)+10 log₁₀ (2)[dB]. In the remaining cases, inassociation with an arbitrary modulation method and the number ofarbitrary layers, ρ_(A) may be denoted by ρ_(A)=P_(A)+Δ_(offset)[dB].Δ_(offset) is given by a nomPDSCH-RS-EPRE-Offset parameter configured byhigher layer signaling.)

Definition of the above-mentioned assumptions may indicate that CQIincludes not only information regarding channel quality but also variousinformation of a corresponding UE. That is, different CQI indexes may befed back according to a throughput or performance of the correspondingUE at the same channel quality, so that it is necessary to define apredetermined reference for the above-mentioned assumption.

Conventional RLM/RRM measurement on a serving cell is performed using aCRS. However, since precoding is applied in a transmission mode (e.g.,transmission mode 9) using a DMRS, the RLM/RRM measurement may bedifferent from measurement on link in which actual transmission isperformed. Accordingly, when a PMI/RI reporting mode is configured intransmission mode 9, the UE performs channel measurement in order tocalculate a CQI value based on a CSI reference signal only. On the otherhand, when the PMI/RI reporting mode is not configured in transmissionmode 9, the UE performs channel measurement for CQI calculation based onthe CRS.

A procedure in which the UE recognizes a channel state to obtain aproper MCS may be designed in various ways for embodiment of the UE. Forexample, the UE may calculate a channel state or validsignal-to-interference plus noise ratio (SINR) using a reference signal.In addition, the channel state or the valid SINR can be measured on anentire system bandwidth (which is referred to as set S) or on a partialbandwidth (specific subband or specific RB). CQI of the entire systembandwidth (set S) may be referred to as a wideband (WB) CQI and CQI ofthe partial bandwidth may be referred to as a subband (SB) CQI. The UEmay obtain the highest MCS based on the calculated channel state orvalid SINR. The highest MCS refers to MCS satisfying the assumption ofthe CQI calculation in which a decoding transfer block error rate doesnot exceed 10%. The UE may determine a CQI index associated with thecalculated MCS and report the determined CQI index to the eNB.

In an LTE/LTE-A system, CSI reference resource for CSI feedback/reportis defined. The CSI reference resource is defined as a group of DLphysical resource blocks (PRBs) corresponding to a frequency bandassociated with the calculated CQI in the frequency domain. In addition,the CSI reference resource is defined as a single DL subframe n-n_(CQI)_(_) _(ref) in the time domain. Here, n is a UL subframe index for CSItransmission/report.

In the case of periodic CSI reporting, n_(CQI) _(_) _(ref) has asmallest value corresponding to a valid DL subframe among values equalto or more than 4. That is, n_(CQI) _(_) _(ref) corresponds to a validDL subframe that is most close to a UL subframe for CSI reporting amongat least 4^(th) previous subframes in a UL subframe for CSI reporting.In addition, in the case of aperiodic CSI reporting, the CSI referenceresource may be the same as a valid DL subframe in which correspondingCSI request in UL DCI format (e.g., DCI format 0) is transmitted. Inaddition, in the aperiodic CSI reporting, when the corresponding CSIrequest is transmitted in random access response grant in the DLsubframe n-n_(CQI) _(_) _(ref), n_(CQI) _(_) _(ref) is 4.

In addition, when CSI subframe sets (C_(CSI,0), C_(CSI,1)) areconfigured for a corresponding UE by a higher layer, each CSI referenceresource may be included in any one of two subframe sets (C_(CSI,0),C_(CSI,1)) but cannot be included in the both subframes.

A DL subframe can be valid if i) it is configured as a DL subframe for acorresponding UE, ii) it is not a multicast-broadcast single frequencynetwork (MBSFN) subframe except for transmission mode 9, iii) it doesnot contain a DwPTS field when a length of the DwPTS in a specialsubframe of a TDD system is equal to or less than a predeterminedlength, iv) it is not contained in a measurement gap configured for thecorresponding UE, and vi) it is an element of the CSI subframe setassociated with the periodic CSI report when the UE is configured withCSI subframes sets for periodic CSI reporting. On the other hand, ifthere is not valid DL subframe for the CSI reference resource, CSIreporting is omitted in UL subframe n.

3) Radio Resource Management (RRM)

Measurement for RRM may be largely classified into reference signalreceived power (RSRP), reference signal received quality (RSRQ), etc.,and the RSRQ may be measured via a combination of RSRP and E-UTRAcarrier received signal strength indicator (RSSI).

The RSRP is defined as a linear average of power distribution ofresource elements in which a cell-specific reference signal (CRS) istransmitted in a measurement frequency band. For RSRP determination, acell-specific reference signal (R0) corresponding to antenna port ‘0’may be used. For RSRP determination, a cell-specific reference signal(R1) corresponding to antenna port ‘1’ may be further used. Whenreception diversity is used by the UE, the reported value may not besmaller than the corresponding RSRP of individual diversity branch. ForRSRP determination, a measurement frequency band used by the UE and thenumber of resource elements used in a measurement period may bedetermined by the UE as long as corresponding accuracy requirements aresatisfied. In addition, power per resource element may be determinedfrom energy from a portion of a symbol except for a cyclic prefix (CP).

Reference signal received quality (RSRQ) is defined as N×RSRP/E-UTRAcarrier received signal strength indicator (RSSI). Here, N is the numberof resource blocks (REs) of an E-UTRA carrier RSSI measurement band. Inaddition, in the aforementioned formula, measurement of the numeratorand the denominator may be achieved from a set of the same RB set.

The E-UTRA carrier RSSI includes a linear average of total receptionpower detected from all sources including a serving cell and non-servingcell of a co-channel, adjacent channel interference, thermal noise, etc.in OFDM symbols containing a reference symbol corresponding to antennaport ‘0’ over N resource blocks in a measurement band. On the otherhand, when specific subframes for performing RSRQ measurement areindicated via higher layer signaling, the RSSI is measured via all OFDMsymbols in the indicated subframes. When reception diversity is used bythe UE, the reported value may not be smaller than the correspondingRSRP of individual diversity branch.

2. Channel State Information Measurement Method

The present invention proposes an effective interference measurementmethod for calculation of channel state information (CSI) by a UE in asituation in which an eNB dynamically changes the amount of UL resourceand DL resource according to the volume of UL and DL traffic. Here,resource configured as UL resource refers to a UL band in an FDD systemand refers to a UL subframe in a TDD system. On the other hand, resourceconfigured as DL resource refers to a DL band in an FDD system andrefers to a DL subframe in a TDD. In addition, the dynamic change in theamount of UL and DL resources means that resource configured as ULresource is temporarily used for DL transmission when the volume of DLtraffic is high, or vice versa. For example, when an eNB notifies aplurality of un-specific UEs of information indicating that a specificsubframe is configured as a UL subframe, if the volume of DL traffic ishigh, the eNB may temporarily notify a specific UE of informationindicating that the corresponding subframe is converted to be used forDL transmission.

In an environment in which the amount of UL and DL resources isdynamically changed, since it is possible to potentially perform DLtransmission on even UL resource, an UE needs to calculate a CRI for ULresource on which DL transmission is performed and to report thecalculated CSI to an eNB. As described above, the UE can measure areception signal when the UE calculates the CSI. In this regard, forSINR calculation, signal components and interference components (orinterference or noise components) need to be estimated. That is, the CSIneeds to be calculated based on interference observed by the UE in ULresource in which DL transmission can be performed, and thus, the eNBneeds to appropriately determine resource used by the UE so that the UEcan appropriately perform the aforementioned interference measurement.

Hereinafter, for convenience of description, a detailed description hasbeen given assuming that the eNB temporarily uses resource configured asUL resource for DL transmission. However, embodiments of the presentinvention are not limited thereto. That is, the present invention canalso be applied to the case in which resource configured as DL resourcemay be temporarily used for UL transmission. In addition, it is assumedthat a boundary of UL/DL subframes with an adjacent cell is aligned.

FIG. 6 is a diagram illustrating an example of a channel stateinformation measuring method according to an embodiment of the presentinvention.

Referring to FIG. 6, an eNB configures interference measurement resourcefor measurement of interference as background information for reportingCSI by a UE in UL resource (S601). The eNB may receive information aboutchange in use of a subframe of the corresponding eNB to DL from UL fromthe eNB of an adjacent cell to configure interference measurementresource. That is, the eNB can configure interference measurementresource for a specific UE in subframes, change in use of which is notserious, such that the corresponding eNB can more accurately measureinterference from an adjacent cell. Step S601 will be described indetail in 2. 3. below.

The interference measurement resource can be fixedly configured and inthis case, step S601 may be omitted.

The eNB transmits interference measurement resource information formeasurement of interference by the configured UE to the UE (S603). Here,the interference measurement resource information may include locationinformation, etc. of the configured interference measurement resource,and the interference measurement resource may be semi-statically ordynamically configured and transmitted via a physical layer signal or ahigher layer signal. Step S603 will be described in detail in 2. 1.below.

Then the eNB transmits, to the UE, interference measurement resourceindication information indicating whether corresponding interferencemeasurement resource on which the UE will perform interferencemeasurement is valid among configured interference measurement resources(S605). Here, when the interference resource is semi-staticallyconfigured, the interference measurement resource indication informationmay indicate whether interference measurement resource contained in aspecific subframe is valid. However, when the interference measurementresource is dynamically configured, the indication information may notbe transmitted to the UE, and the UE may implicitly consider that theconfigured interference measurement resource is valid through theinterference measurement resource information in step S603. In thiscase, step S605 may be omitted. Step S605 will be described in detail in2. 2. below.

The UE measures interference from an adjacent cell in valid interferencemeasurement resource (S607) and calculates CSI using the measuredinterference (S609). Here, a period in which the UE measures the CSI maybe limited to one subframe and may include a plurality of subframes.When the CSI measurement period includes a plurality of subframes, theUE may calculate an average value of the CSI measured every subframe orwith a predetermined period in the corresponding period.

Steps S607 and S609 will be described in detail in 2. 1. and 2. 2.below.

Then the UE reports the calculated CSI to the eNB (S611). As describedabove, when the CSI measurement period contains a plurality ofsubframes, the UE may report an average value of the calculated CSI tothe eNB.

When the eNB dynamically changes use of resource configured as UL (orDL) resource for UL resource and DL resource and uses the configuredresource, scheduling restriction can be limited in Tx-Rx switching timein which resource used for UL and resource used for DL are changed. Forexample, when a next subframe (n+1) of a specific subframe (n) is usedfor DL in resource configured as UL resource (or when a next subframe isnot scheduled for UL transmission), scheduling may be limited in a lastsymbol of the corresponding subframe (n). In addition, in the subframe(n+1) used for DL, scheduling may also be limited in a last symbol ofthe corresponding subframe (n+1).

Hereinafter, a channel state information measurement method according tothe present invention will be described.

2. 1. Interference Measurement Resource Information

Interference measurement resource refers to resource for measurement ofinterference as background information for reporting CSI by a UE in anenvironment in which the amount of UL and DL resources is dynamicallychanged. An eNB may transmit information about interference measurementresource to the UE via a higher layer signal (e.g., a radio resourcecontrol (RRC) layer or a media access control (MAC) layer signal) or aphysical layer signal so as to notify the UE of resource in which the UEneeds to measure interference to DL transmission in UL resource.

The interference measurement resource information may include locationinformation for notifying the UE of a location of resource configured asinterference measurement resource. Here, the location of theinterference measurement resource may be determined according to atleast one of offset information of a subframe, period information of asubframe, and subcarrier or OFDM/SC-FDMA symbol index information.

The location information of the interference measurement resource mayinclude offset information of a subframe. That is, the eNB may determineonly a specific DL subframe in which the UE will perform DL measurementamong resources used for DL in resource configured as UL resource ofresources configured as UL resources and notify the UE of the determinedsubframe through the subframe offset information. Here, the determinedsubframe may be one or more subframes. For example, when the eNBtransmits the location information of the interference measurementresource to the UE, the offset information may indicate the determinedsubframe based on a subframe in which the location information of theinterference measurement resource is transmitted. In addition, theoffset information may be expressed as index information of thesubframe.

The location information of the interference measurement resource mayinclude period information of the subframe. That is, the eNB maydetermine only DL subframes with a specific period in which the UE willperform DL measurement among resources used for DL in resourcesconfigured as UL resources and notify the UE of the determined subframethrough the subframe period information. Here, the subframe periodinformation may indicate a period of a subframe for interferencemeasurement in a unit of a integer multiple of one or more radioframes/half frames/subframes.

In addition, the location information of the interference measurementresource may include OFDM/SC-FDMA symbol and/or subcarrier indexinformation. That is, the eNB may determine only a specific OFDM symboland/or specific subcarrier of a random DL subframe in which the UE willperform DL measurement among resources used for DL in resourcesconfigured as UL resources. Here, the number of the determined OFDMsymbol and/or subcarrier may be one or more.

The eNB may use the aforementioned information alone or combine and useone or more pieces of the information in order to notify the UE of thelocation of the interference measurement resource determined forinterference measurement of the UE. For example, the UE is configured toperform interference measurement in all OFDM symbols of a subframe witha specific period, the eNB may transmit the location information of theinterference measurement resource including subframe period and offsetinformation only to the UE, and when the UE is configured to performinterference measurement using a specific resource element of a specificsubframe, the eNB may transmit the location information of theinterference measurement resource including subframe offset and/orsymbol/subcarrier index information only to the UE.

The aforementioned interference measurement resource needs to have theform of a signal present in a legacy 3GPP LTE/LTE-A system. This isbecause existing signaling format can be used and a signal that aspecific UE wants to avoid in interference measurement can be easilyexcluded. For example, when the interference measurement resource isactually used for UL, a location of resource for interferencemeasurement having a specific signaling format form to corresponding UEsand nulling of a signal may be indicated at the corresponding locationin order to exclude the signal of the specific UE. In addition, when theinterference measurement resource is actually used for DL, the eNB maynotify UEs that receive a DL signal that a specific UE wants to avoid ininterference measurement of the UE of the location of resources forinterference measurement having a specific signaling format form andindicate that a meaningful signal is not transmitted to the eNB at thecorresponding location.

1) The interference measurement resource may have a form of a ULtransmission signal, which can be more effectively used in that aninterference measurement operation is performed in a region configuredas UL resource on existing UEs.

As an example of the interference measurement resource, the eNB mayconfigure a specific SRS or DMRS and indicate interference measurementto be performed at a resource location of the corresponding referencesignal. That is, when the eNB wants specific UL resource to be used forDL transmission, the eNB may transmit configuration information of thespecific SRS or DMRS to UEs that are targets of the operation andcommand to the UEs to measure interference at the resource location ofthe corresponding reference signal and to report CSI of resource to beused for DL transmission. That is, the aforementioned interferencemeasurement resource information may include configuration informationof reference signal. An example of the configuration information of thereference signal may include sequence information of an SRS or DMRSconfigured in the interference measurement resource, cyclic shiftinformation of the sequence of the configured SRS or DMRS, spreadingcode information, frequency shift information, etc. In addition, theconfiguration information of the reference signal needs to be fixedlyconfigured so as to be previously known to both the eNB and the UE.

The eNB notifies the UE of information indicating that zero power isadditionally supplied to SRS or DMRS for interference measurementthrough the interference measurement resource information such that thecorresponding UE can directly measure interference between cells exceptfor cell interference in the corresponding resource. That is, the UE canmeasure interference of adjacent cells based on a region to which zeropower is supplied. In particular, in the case of DMRS, since a pluralityof DMRSs can be code division multiplexed (CDM) in the samefrequency-time resource, the UE may perform despreading in interferencemeasurement resource in which the DMRS is transmitted using a CDMsequence (or a spreading code) of a specific DMRS configured forinterference measurement (or supply of zero power may be configured),perform nulling on a sequence that is actually used as the DMRS, andthen operate to measure observed remaining interference. Here, the eNBmay allocate a sequence that is not used as a DMRS configured forinterference measurement in an adjacent cell such that the UE mayoperate to measure interference between cells except for cellinterference in the corresponding resource.

2) The interference measurement resource may have a form of a DLtransmission signal, which is advantageous in that correspondingresource has a signal format appropriate for actual use thereof so as tosmoothen an operation of a UE with capability of understanding the useconversion. That is, when interference measurement resource is actuallyused for DL, the UE have the same signal format for channel states fromthe eNB, and thus, can measure a DL signal or interference of adjacentcells according to use (UL or DL) of the corresponding resource using aformat of the same interference measurement resource.

As an example of the interference measurement resource, the eNB mayconfigure a specific CRS or CSI-RS and indicate interference measurementto be performed at a resource location of the corresponding referencesignal. That is, when the eNB wants specific UL resource to be used forDL transmission, the eNB may transmit configuration information of thespecific CRS or CSI-RS to UEs that are targets of the operation andcommand to the UEs to measure interference at the resource location ofthe corresponding reference signal and to report CSI of resource to beused for DL transmission. As described above, the aforementionedinterference measurement resource information may include configurationinformation of reference signal. The configuration information of thereference signal may include sequence information of a CRS or CSI-RSconfigured in the interference measurement resource, cyclic shiftinformation of the sequence of the configured CRS or CSI-RS, spreadingcode information, frequency shift information, etc. In addition, theconfiguration information of the reference signal needs to be fixedlyconfigured so as to be previously known to both the eNB and the UE.

Like in the UL transmission signal form, the eNB notifies the UE ofinformation indicating that zero power is additionally supplied to CRSor CSI-RS for interference measurement through the interferencemeasurement resource information such that the corresponding UE candirectly measure interference between cells except for cell interferencein the corresponding resource. That is, the UE can measure interferenceof adjacent cells based on a region to which zero power is supplied.

2. 2. Determination of Valid Interference Measurement Resource

As described in 2. 1. above, the interference measurement resource maybe semi-statically configured using a higher layer signal such as an RRCsignal. As described above, this is because the location information ofthe interference measurement resource includes many configurationparameters such as the subframe resource, offset, subcarrier, and/orsymbol index of the corresponding resource, and thus, overhead is toohigh to use a physical layer signal.

As described above, the eNB uses a specific subframe for DL or ULaccording to a traffic situation and dynamically determines use of thecorresponding subframe, a subframe including semi-statically configuredinterference measurement resource may also be dynamically determined.This causes issues in terms of interference measurement. For example,when the eNB commands the UE to perform interference measurement using aspecific CSI-RS configuration, if the eNB uses a subframe in whichcorresponding interference measurement resource is present for UL and anadjacent UE in the same cell transmits a PUSCH, a strong signal isobserved from the corresponding interference measurement resource. ThePUSCH signal of the UE in the same cell corresponds to interference thatdoes not actually occur when the corresponding subframe is used for DL,and thus needs to be excluded from interference measurement. Thus, whenuse of a subframe is dynamically changed, an operation for appropriatelyusing interference measurement resource that is semi-staticallyconfigured is also required.

Likewise, when the use of the subframe is dynamically changed, the eNBmay transmit indication information indicating whether interferencemeasurement resource is valid to the UE in order to notify the UE ofwhether semi-statically configured interference measurement is valid.The indication information may be classified as following according touse of a subframe including the interference measurement resource.

Hereinafter, for convenience of description, it is assumed that UE(s)connected to the same cell as a corresponding UE except for a UE thatintends to perform interference measurement know a location ofinterference measurement resource.

2. 2. 1. When a Subframe Including Interference Measurement Resource isUsed for UL

FIG. 7 is a schematic diagram illustrating a case in which a subframeincluding interference measurement resource is used for UL according toan embodiment of the present invention.

Referring to FIG. 7, when an eNB of a Cell 1 semi-statically configuresinterference measurement resource for measuring interference by aspecific UE UE1 within coverage, since use of a subframe is dynamicallychanged, a subframe including interference measurement resource is usedas a uplink subframe (UL SF) and the corresponding subframe is also usedas a UL SF in an adjacent cell Cell 2.

Operations of UEs are now described. The UE 1 does not performscheduling for UL transmission and measures DL interference in asubframe including interference measurement resource. In addition, theUE 2 belonging to the same cell as the UE 1 transmits UL data to an eNBof the Cell 1 through a PUSCH in a subframe including the interferencemeasurement resource due to UL scheduling. A UE 3 that does not belongto the same cell coverage as the UE 1 also transmits UL data through aPUSCH to an eNB of the Cell 2 in a subframe including the interferencemeasurement resource due to UL scheduling.

When a subframe including interference measurement resource, in which aspecific UE performs interference measurement, is used for UL, other UEs(in particular, UEs connected to the same cell) may perform nulling onthe corresponding interference measurement resource during ULtransmission in the corresponding subframe. That is, in FIG. 7, when theUE 1 measures interference in specific resource, the UE 2 connected tothe same cell may perform nulling on a location of interferencemeasurement resource in a transmission location of a PUSCH of the UE 1and apply rate matching to data to be transmitted through the PUSCH. Inaddition, during a CoMP operation, the UE 3 may also perform nulling alocation of the interference measurement resource in the PUSCHtransmission region and apply rate matching to data to be transmittedthrough the PUSCH. Likewise, other UEs UE 2 and/or UE 3 that do notmeasure interference may perform nulling UL data at the location of theinterference measurement resource, and thus, the UE 1 can moreaccurately measure interference.

As described above, the interference measurement resource may have thesame DL transmission pattern such as a CRS or a CSI-RS or the same ULpattern such as an SRS or a DMRS. In this case, other UEs UE 2 and/or UE3 may perform nulling a transmission of a PUSCH according to atransmission pattern of a DL RS or a pattern of a UL RS and operate toperform rate matching on data to be transmitted through a PUSCH. Inaddition, as described above, when the interference measurement resourcehas the same pattern as a DMRS, the UE 1 may perform dispreading in aninterference measurement resource region using a configured DMRS CDMsequence such that other UEs can perform nulling on the DMRS sequencethat is actually used as a reference signal and then measure remaininginterference.

Likewise, when a subframe including the interference measurementresource is used for UL, if a UE in the same cell does not performnulling on the interference measurement resource, appropriateinterference measurement is not performed. To address this issue, an eNBmay transmit indication information indicating whether interferencemeasurement resource positioned in a corresponding subframe is valid toa UE via a physical layer signal or a MAC layer signal. Here, theindication information may be configured in the form of an indicatorindicting whether nulling is performed on the location of theinterference measurement resource in the PUSCH transmission region byother UEs. In addition, the indication information may be transmittedevery subframe including the interference measurement resource or may beconfigured in the form of a bitmap for one or more subframes andtransmitted with a specific period. As described above, when a CSImeasurement period includes a plurality of subframes, the indicationinformation may include information about the number of valid subframescontained in the corresponding period.

The UE may consider that interference measurement resource is valid onlyupon receiving the indication information indicating that nulling isperformed on the interference measurement resource region in a PUSCHtransmission region of other UEs. That is, the UE may measureinterference in the valid interference measurement resource only andomit interference measurement in invalid interference measurementresource.

2. 2. 2. When a Subframe Including Interference Measurement Resource isUsed for DL

FIG. 8 is a schematic diagram illustrating a case in which a subframeincluding interference measurement resource is used for DL according toan embodiment of the present invention.

Referring to FIG. 8, when an eNB of a Cell 1 semi-statically configuresinterference measurement resource for measuring interference by aspecific UE UE1 within coverage, since use of a subframe is dynamicallychanged, a subframe including interference measurement resource is usedas a downlink subframe (DL SF) and the corresponding subframe is alsoused as a UL SF in an adjacent cell Cell 2.

Operations of UEs are now described. The UE 1 measures DL interferencein a subframe including interference measurement resource. In addition,the UE 2 belonging to the same cell as the UE 1 receives DL data from aneNB (Cell 1) through a PDSCH in a subframe including the interferencemeasurement resource due to DL scheduling. A UE 3 that does not belongto the same cell coverage as the UE 1 transmits UL data through a PUSCHto an eNB (Cell 2) in a subframe including the interference measurementresource due to UL scheduling.

Like in FIG. 8, the UE may consider the aforementioned interferencemeasurement resource that is semi-statically configured as potentialinterference measurement resource and determine the interferencemeasurement resource as valid interference measurement resource onlywhen a corresponding UL subframe is actually used for DL transmission.For example, even if an eNB configures interference measurement resourceto a specific UE via a higher layer signal, when the eNB schedules ULtransmission to the corresponding UE in a corresponding subframe, thecorresponding UE needs to perform the signal transmission operation andthus cannot perform interference measurement. Accordingly, in this case,interference measurement resource in which a higher layer signal isconfigured is considered to be invalid, and a measured value at acorresponding point in time needs to be excluded from CSI calculation.

Likewise, when a subframe including interference measurement resource inwhich a specific UE performs interference measurement is actually usedfor UL, the eNB may not appropriately perform interference measurement.To address this issue, an eNB may transmit indication informationindicating whether interference measurement resource positioned in acorresponding subframe is valid to a UE via a physical layer signal or aMAC layer signal. Here, the indication information may be configured inthe form of an indicator indicting whether the corresponding subframe isused for DL or UL. In addition, the indication information may betransmitted every subframe including the interference measurementresource or may be configured in the form of a bitmap for one or moresubframes and transmitted with a specific period. As described above,when a CSI measurement period includes a plurality of subframes, theindication information may include information about the number of validsubframes contained in the corresponding period.

The UE may consider that interference measurement resource is valid onlyupon receiving the indication information indicating that a specificsubframe including interference measurement resource is actually usedfor DL. That is, the UE may measure interference in the validinterference measurement resource only and omit interference measurementin invalid interference measurement resource.

In addition, an indicator for triggering aperiodic CSI reporting may bean indicator indicating validity of interference measurement resource.For example, CSI request information contained in a UL DCI format can beused. That is, when aperiodic CSI reporting is triggered in a specificsubframe or a specific subframe is determined as valid referenceresource about aperiodic CSI reporting triggered in another subframe,the UE may consider that the interference measurement resource includedin the corresponding subframe is valid, measure interference, and usethe interference for CSI calculation. That is, when aperiodic CSIreporting is not triggered in a specific subframe or a specific subframeis not determined as valid reference resource about aperiodic CSIreporting triggered in another subframe, the UE consider that theinterference measurement resource included in the corresponding subframeis not valid. In order to prevent a measured value of the validinterference resource and a measured value of the invalid interferenceresource from being mixed or to correspond to errors of receiving theindication information about whether interference measurement resourceis valid by the UE, the UE may omit a procedure of combining withinterference measured in another subframe and operate to use onlyinterference measured in the corresponding subframe during calculationof CSI based on interference measured in a subframe determined as a ULsubframe.

2. 3. Interference Measurement Resource Configuration and InterferenceAdjustment

As described above, when a UE measures adjacent cell interference, theadjacent cell dynamically changes use of a subframe includinginterference measurement resource, interference measured by thecorresponding UE may not be uniformly maintained. For example, when anadjacent uses a subframe of subframes including interference measurementresource for PUSCH transmission of a UE of the corresponding cell butuses another subframe for PDSCH transmission of an eNB, since atransmission object (an eNB or a UE) is changed in each subframe,spatial characteristics (e.g., interference covariance matrix) ofinterference as well as an overall interference power level is alsochanged, and thus, it may be difficult to accurately measureinterference.

To address the aforementioned issue, an eNB of each cell may transmitinformation about change in use of a subframe to an eNB of an adjacentcell. That is, an eNB of each cell may transmit a message including anindex of a UL subframe to be used for DL transmission or UL transmissionwith very high possibility to an eNB of the adjacent cell. An eNB thatreceives the message from the eNB of the adjacent cell may intensivelyconfigure interference measurement resource in a subframe determined bythe corresponding message to prevent excessive change in theinterference measurement due to the aforementioned use of the subframe.Here, the eNB may compare a specific threshold with calculatedpossibility and notify an eNB of an adjacent cell of an index of asubframe having possibility that is equal to or more than the specificthreshold.

In addition, there is a method of scheduling such that an eNB uniformlymaintains a changing degree of interference that affects an adjacentcell in a specific subframe if possible. For example, when the eNB usesa specific UL subframe for DL transmission, the eNB may appropriatelyadjust transmission power of the eNB and may operate such that theattribute of interference to the adjacent cell is similar to a level ofinterference affected by UL transmission of the UE present in a cell ofthe eNB.

Meanwhile, the adjacent cell can measure how long consistency ofinterference in the cell is maintained. For example, the UE may measurechangeability of an interference level of the same interferencemeasurement resource and periodically or aperiodically (e.g., whenchange in the interference level exceeds a predetermined level) reportthe measured changeability to an eNB. Here, changeability information ofthe interference level may include a maximum interference level, aminimum interference level, a difference between the maximum and theminimum, an index of resource on which a predetermined level or more ofinterference is exerted, etc. Likewise, the eNB that receives thechangeability information of the interference level from the UE maytransmit a message for requesting an eNB of an adjacent cell touniformly maintain the interference level to the eNB of the adjacentcell. The message for requesting of the uniform interference level mayinclude a changing degree (the maximum, the minimum, the differencetherebetween, etc.) of the interference level, information about whethera desired level or more interference is exerted, information about anindex of resource on which a desired level or more interference isexerted, etc.

The eNB may fluidly operate according to the message for requesting auniform interference level, received from the adjacent cell.

That is, when the eNB transmits an index of a subframe to be used for DLtransmission or UL transmission of a UL subframe with very highpossibility to an eNB of the adjacent cell, a threshold as a referencefor calculation of the possibility may be dynamically changed accordingto reception of the message for requesting a uniform interference levelfrom the adjacent cell. For example, upon receiving the message forrequesting the uniform interference level from an eNB of an adjacentcell or receiving a corresponding message a predetermined number oftimes or more for a predetermined period of time, the eNB may determinethat change in interference exerted on the adjacent cell is high andchange the threshold to a high threshold, and when the eNB does notreceive a message for requesting an interference level for apredetermined period of time, the eNB may determine that change ininterference exerted on the adjacent cell is low and change thatthreshold to a high threshold.

In addition, when a UL subframe is used for DL transmission, adjustmentof transmission power may be dynamically changed according to receptionof the message for requesting the uniform interference level from theadjacent cell. For example, upon receiving the message for requestingthe uniform interference level from an eNB of an adjacent cell orreceiving a corresponding message a predetermined number of times ormore for a predetermined period of time, the eNB may determine thatchange in interference exerted on the adjacent cell is high and changethe threshold to a low threshold, and when the eNB does not receive amessage for requesting an interference level for a predetermined periodof time, the eNB may determine that change in interference exerted onthe adjacent cell is low and increase transmission power.

4. Overview of Apparatus to which the Present Invention is Applicable

FIG. 9 is a block diagram of a wireless communication apparatusaccording to an embodiment of the present invention.

Referring to FIG. 9, a wireless communication system includes a BS 90and a plurality of UEs 100 positioned within a region of the BS 90.

The BS 90 includes a processor 91, a memory 92, and a radio frequency(RF) unit 93. The processor 91 embodies the proposed functions,processes, and/or methods. Layers of a wireless interface protocol maybe embodied by the processor 91. The memory 92 is connected to theprocessor 91 and stores various pieces of information for driving theprocessor 91. The RF unit 93 is connected to the processor 91 andtransmits and/or receives a radio signal.

The UE 100 includes a processor 101, a memory 102, and an RF unit 103.The processor 101 embodies the proposed functions, processes, and/ormethods. Layers of a wireless interface protocol may be embodied by theprocessor 101. The memory 102 is connected to the processor 101 andstores various pieces of information for driving the processor 101. TheRF unit 103 is connected to the processor 101 and transmits and/orreceives a radio signal.

The memories 92 and 102 may be disposed within or outside the processors91 and 101 and may be connected to the processors 91 and 101 via variousmeans. In addition, the BS 90 and/or the UE 100 may have a singleantenna or multiple antennas.

The embodiments of the present invention described above arecombinations of elements and features of the present invention. Theelements or features may be considered selective unless otherwisementioned. Each element or feature may be practiced without beingcombined with other elements or features. Further, an embodiment of thepresent invention may be constructed by combining parts of the elementsand/or features. Operation orders described in embodiments of thepresent invention may be rearranged. Some constructions of any oneembodiment may be included in another embodiment and may be replacedwith corresponding constructions of another embodiment. It is obvious tothose skilled in the art that claims that are not explicitly cited ineach other in the appended claims may be presented in combination as anembodiment of the present invention or included as a new claim by asubsequent amendment after the application is filed.

The embodiments of the present invention may be achieved by variousmeans, for example, hardware, firmware, software, or a combinationthereof. In a hardware configuration, the methods according to exemplaryembodiments of the present invention may be achieved by one or moreapplication specific integrated circuits (ASICs), digital signalprocessors (DSPs), digital signal processing devices (DSPDs),programmable logic devices (PLDs), field programmable gate arrays(FPGAs), processors, controllers, microcontrollers, microprocessors,etc.

In a firmware or software configuration, an embodiment of the presentinvention may be implemented in the form of a module, a procedure, afunction, etc. Software code may be stored in a memory unit and executedby a processor. The memory unit is located at the interior or exteriorof the processor and may transmit and receive data to and from theprocessor via various known means.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the present inventionwithout departing from the spirit or scope of the invention. Thus, it isintended that the present invention cover the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

INDUSTRIAL APPLICABILITY

The data transmission/reception method in a wireless access systemaccording to the present invention has been described in terms of anexample applied to a 3^(rd) generation partnership project long termevolution (3GPP LTE) system but can be applied to various wirelessaccess systems in addition to the 3GPP LTE system.

The invention claimed is:
 1. A method for obtaining channel stateinformation in a wireless access system for supporting traffic adaption,the method performed by a user equipment (UE) and comprising: receivinginterference measurement resource information comprising locationinformation of interference measurement resource; receiving uplink (UL)and downlink (DL) configuration information indicating whether one ormore UL subframes among UL resources are reconfigured to one or more DLsubframes for the traffic adaption; and measuring interference on theinterference measurement resource included in the one or more DLsubframes being reconfigured based on the UL and DL configurationinformation, wherein the location information of the interferencemeasurement resource include a subframe offset, a subframe period, andwherein a location of the interference measurement resource is specifiedusing a combination the subframe offset and the subframe period.
 2. Themethod of claim 1, wherein the interference measurement resource has aform of zero-power channel state information reference signal (CSI-RS)resource configuration.
 3. The method of claim 1, wherein theinterference measurement resource is configured via higher layersignaling.
 4. The method of claim 1, wherein the first UL and DLconfiguration is indicated by Physical Downlink Control Channel (PDCCH).5. The method of claim 1, the method further comprising: calculating thechannel state information using the measured interference; andtransmitting the calculated channel state information to an eNB (evolvedNode B).
 6. The method of claim 1, wherein the UL and DL configurationinformation indicating whether one or more UL subframes are reconfiguredis dynamic control information, and wherein the UL resources isconfigured by system information.
 7. A user equipment (UE) for measuringchannel state information in a wireless access system for supporting anenvironment in which amounts of uplink (UL) and downlink (DL) resourcesare dynamically changed, the UE comprising: a radio frequency (RF) unitfor transmitting the channel state information and for transmitting andreceiving a radio signal; and a processor which is configured to:receive interference measurement resource information comprisinglocation information of interference measurement resource, receiveuplink (UL) and downlink (DL) configuration information indicatingwhether one or more UL subframes among UL resources are reconfigured toone or more DL subframes for the traffic adaption, and measureinterference on the interference measurement resource included in theone or more DL subframes being reconfigured based on the UL and DLconfiguration information, wherein the location information of theinterference measurement resource include a subframe offset, a subframeperiod, and wherein a location of the interference measurement resourceis specified using a combination the subframe offset and the subframeperiod.
 8. The UE of claim 7, wherein the interference measurementresource has a form of zero-power channel state information referencesignal (CSI-RS) resource configuration.
 9. The UE of claim 7, whereinthe interference measurement resource is configured via higher layersignaling.
 10. The UE of claim 7, wherein the first UL and DLconfiguration is indicated by Physical Downlink Control Channel (PDCCH).11. The UE of claim 7, the processor further configured to calculate thechannel state information using the measured interference and transmitthe calculated channel state information to an eNB (evolved Node B). 12.The UE of claim 7, wherein the UL and DL configuration informationindicating whether one or more UL subframes are reconfigured is dynamiccontrol information, and wherein the UL resources is configured bysystem information.